(a) Field of the Invention
The present invention relates to a high contrast ratio membrane mask and, more particularly, to a membrane mask for use in an electron beam lithography or an X-ray lithography. The present invention also relates to a method for forming such a membrane mask.
(b) Description of the Related Art
A membrane mask is used in an electron beam lithography and an X-ray lithography during a fabrication process for fabricating a semiconductor device. The membrane mask generally includes a mask body pattern used as a scattering film for scattering electron beams or an absorbing film for absorbing X-rays, a membrane film for supporting the mask body pattern, and a silicon substrate for supporting the membrane film.
A conventional technique for forming a membrane mask will be described first with reference to FIGS. 1A to 1C. A boron nitride film 42 having a thickness of 3 xcexcm is deposited on a silicon wafer 41 by a CVD technique, followed by deposition of a tungsten film 43 thereon by using a high-frequency sputtering technique, as shown in FIG. 1A. The tungsten film 43 has a thickness around 1.5 xcexcm and has a mixed-phase crystal structure including an xcex1 phase and xcex2 phase.
Subsequently, as shown in FIG. 1B, a resist film 44 for an electron beam exposure is applied thereon by spin-coating, followed by exposure of an electron beam 45 to write a desired pattern on the resist film 44.
Thereafter, as shown in FIG. 1C, the resist film 44 is developed to form an electron beam mask having a desired pattern. Then, as shown in FIG. 1D, the tungsten film 43 is subjected to a reactive ion etching (RIE) process using the resist film 44 as a mask to obtain a membrane mask having the desired pattern.
In the conventional process as described above, it is known that the mixed-phase crystal structure of the tungsten film 43 including the xcex1 phase and the xcex2 phase affords a smaller internal stress of the tungsten film 43. The smaller internal stress allows the membrane mask to have a lower distortion in the mask pattern if the membrane mask has a larger thickness for achieving a higher contrast ratio.
Recently, it is desired that the membrane mask have a higher patterning accuracy as well as a higher locational accuracy in view of the rapid development of the finer design rule in a semiconductor device. It is known that the tungsten film has a large internal stress due to the specific property of the tungsten film known in the heavy metals. The large internal stress causes a film distortion to thereby prevent the patterning accuracy and incurring peeling-off in the membrane mask. The patterning accuracy or prevention of the peeling-off may be achieved by a larger thickness of the tungsten film used as the electron beam scattering film or the X-ray absorbing film.
The larger thickness of the mask body pattern, however, increases the aspect ratio of the mask body pattern, which degrades the patterning accuracy of the mask body film. The aspect ratio is generally defined by a ratio of the film thickness to the width of the pattern on the mask film.
In the conventional technique shown in FIGS. 1A to 1D, the mixed-phase crystal structure of the tungsten film including the xcex1 phase and the xcex2 phase may reduce the film stress, whereby the thickness of the mask body may be reduced. However, the specified contrast ratio desired for the membrane mask prevents the reduction of the film thickness. Thus, the patterning accuracy of the mask body is not achieved in the conventional technique.
In view of the above, it is an object of the present invention to provide a membrane mask having a smaller thickness without degrading the contrast ratio and thus achieving a higher patterning accuracy.
It is also an object of the present invention to provide a method for forming such a membrane mask.
The present invention provides a membrane mask including a wafer, a membrane film including a first material and supported by the wafer, and a mask body overlying the membrane film, the mask body having a mask pattern including an opening, the membrane film having a first area underlying the mask body other than the opening, the first area being formed by addition of atoms having an atomic number higher than an atomic number of the first material.
The present invention also provides a method for forming a membrane mask including the steps of: forming a membrane film supported on a wafer; forming a resist mask on the membrane film; selectively implanting atoms into the membrane film by using the resist mask to form an implanted area, the atoms having an atomic number higher than an atomic number of a material included in the membrane film; forming a mask body film on the membrane film including the implanted area; and patterning the mask body film to have a pattern including an opening exposing the membrane film other than the implanted area.
In accordance with the membrane mask of the present invention and the membrane mask formed by the method of the present invention, the implanted area including the atoms having a higher atomic number has a function of electron beam scattering or X-ray absorbing, thereby assisting the mask body. The implanted area thus improves the contrast ratio of the resultant pattern obtained by the mask body pattern without increasing the thickness of the mask body.